This invention relates to optical fiber Bragg gratings and, in particular, to a fiber Bragg grating provided with a ring of longitudinally extending gap regions (voids) spaced around the core to reduce short wavelength cladding mode loss.
Optical fiber Bragg gratings are critical components in WDM communication systems. They perform several key applications including add/drop filtering, band filtering, and dispersion compensation. In these applications the grating is typically used as a reflective filter. Incident light within the stopband of the grating is strongly reflected whereas light outside the stopband is transmitted. An ideal Bragg grating would possess a rectangular amplitude filter function; the reflection would be unity within the stopband and negligible outside the stopband.
In practice, an important limitation on a realistic optical fiber Bragg grating is cladding mode loss on the short wavelength side of the main reflection band. This short wavelength cladding mode loss is caused by grating induced coupling from the core mode into backward propagating cladding modes. The cladding mode loss is seen in the transmission spectrum as sharp resonances on the short wavelength side of the Bragg resonance. The magnitude of the loss scales approximately with the square of the strength of the grating, and the loss is dramatically exacerbated when many gratings are cascaded. It thus imposes strict limitations on the design of optical networks that use grating based technologies.
Several approaches have been proposed for reducing Bragg grating coupling into claddings. A first approach is to surround the fiber with a lossy polymer material that has a refractive index near that of the glass. In this case the cladding mode extends into the polymer where it is absorbed, and thus core-cladding mode coupling is reduced. The cladding mode spectrum is reduced closer to the radiation limit, typically by a factor of 4-5. This loss is acceptable for many applications but can still limit the number of devices that can be cascaded.
Another approach uses a depressed cladding design proposed by Dong et al. [L. Dong, L. Reekie, J. L. Cruz, J. E. Caplen, J. P. de Sandro and D. N. Payne, xe2x80x9cOptical fibers with depressed claddings for suppression of coupling into cladding modes in fiber Bragg gratings,xe2x80x9d IEEE Photonic Technology Letters, vol. 9, page 64-66 (1997)]. A conventional fiber core is surrounded by a down-doped region, typically using boron to achieve the down doping. The depressed cladding region suppresses the overlap of lower order cladding modes with the core. The transverse oscillations are stretched in the depressed cladding region, since the traverse resonance condition is associated with the optical path length (distance times refractive index). This approach has been demonstrated with moderate success. But it is limited by the amount that the index can be reduced in the depressed cladding region.
A third approach is to increase the offset of the cladding mode loss from the Bragg resonance. This is achieved by increasing core refractive index, such that the core mode effective index is substantially above that of the lowest order cladding mode. The result is that the cladding mode resonances are offset from the Bragg resonance. Various groups have demonstrated this effect, where typically a fiber with xcex94xcx9c2%, and a core diameter of dxcx9c2 xcexcm, is used, resulting in an offset of xcx9c2-5 nm. Although the principle has been demonstrated, the usable bandwidth is still limited by the onset of cladding mode loss. In addition there is a significant penalty incurred due to mode mismatch between the grating fiber and the transmission fiber.
The cladding mode loss can also be reduced by incorporating photosensitive material into the cladding of the fiber (E. Delevaque et al. xe2x80x9cOptical fiber design for strong gratings photoimprinting with radiation mode suppression,xe2x80x9d OFC""95, PD5, USA, 1995 and K. Oh et al., Suppression of cladding mode coupling in Bragg grating using GeO2B2O3 doped photosensitive cladding optical fiber, Electronic Letters, vol. 35, page 423-424 (1999)). In this case, after UV exposure the grating region extends into the cladding region. The reduction in the cladding mode loss follows from the mode orthogonality condition. Hence if the core and the cladding have the same UV sensitivity, there is no blaze and the exposure through the fiber is uniform. Thus the grating will give negligible coupling to the cladding modes. A disadvantage of this scheme is a net reduction in the grating strength due to absorption in the photosensitive cladding region. There is also an increased coupling to asymmetric modes because of the increased asymmetry in the region where these modes have a large mode field strength.
It is also pertinent background that optical fibers have been made with internal longitudinally extending gaps (openings) in the cladding and gratings have been made with such fibers. See U.S. Pat. No. 5,802,236 issued to D. J. DiGiovanni et al. on Sep. 1, 1998 entitled xe2x80x9cArticle Comprising A Microstructured Optical Fiber and Method of Making Such a Fiberxe2x80x9d and U.S. Pat. No. 5,907,652 issued to D. J. DiGiovanni et al. on May 25, 1999 entitled xe2x80x9cArticle Comprising An Air Clad Optical Fiberxe2x80x9dboth of which are incorporated herein by reference. These patents have not addressed the problem of reducing short wavelength cladding mode loss.
Accordingly there is a need for an improved fiber design which can effectively eliminate cladding mode loss in fiber Bragg gratings.
The present invention is predicated on applicants"" discovery that an appropriately spaced and dimensioned internal gap cladding can substantially reduce short wavelength cladding mode loss in a fiber Bragg grating. A fiber Bragg grating is provided with a ring of closely spaced, longitudinally extending gap regions in the glass peripherally surrounding the core. The gaps are spaced apart by thin glass webs having a thickness less than a wavelength of the light being transmitted and are disposed peripherally about the core at a distance of 2-10 wavelengths from the core center. The thin webs limit the passage of the light between the gaps. The combination of webs and gaps acts as an internal thin cladding which supports fewer cladding modes than conventional glass cladding and, significantly, provides increased wavelength spacing between the Bragg resonance and the first cladding mode resonance.